Prions have a new type of inheritance mi which altered phenotypes am based on the inheritance of proteins with self-perpetuating altered conformational states, rather than the inheritance of altered nucleic acids. The Sup35 protein from yeast is normally involved in translation termination. However, under certain conditions it behaves like a prion and forms long fibers. Ile STEM was used to determine the mass/length of different forms of fibers and their intermediates in formation.